The present invention relates generally to inkjet printheads for inkjet printers wherein the printhead includes a plurality of nozzles in fluid communication with an ejection chamber, and ink is ejected from the chamber through the nozzles in drops for printing on a medium. More specifically, this invention pertains to systems or methods for maintaining or recovering nozzle function affected by ink clogging at the nozzles
An inkjet printhead for an inkjet printing system includes a plurality of nozzles through which ink is ejected in drops responsive to printing commands from a controller for printing on a print medium. Whether the printhead is of the type that is permanently mounted on a printing system and linked to an ink source or of a disposable nature that includes a cartridge supporting an ink reservoir, each of the nozzles is disposed on the printhead in fluid communication with an ink ejection chamber. In the case of thermal ink jet printers and printheads, ink is ejected in drops by the application of heat to ink in the ejection chamber responsive to the printing commands. One or more resistive heater is associated with each ejection chamber and generates heat that causes solvents in the ink to vaporize generating bubble in the ejection chamber. The rapid expansion of the bubbles forces ink through the nozzles in drop form.
Other types of printing systems and printheads have a piezoelectric transducer integrated in the printhead forming a wall in the ink ejection chamber, or in some other chamber that that holds ink and is in fluid communication with the ejection chamber. Responsive to printing commands the wall, or the piezoelectric transducer, expands and contracts forcing ink from the ejection chamber in droplet form for printing.
In either of the above-described inkjet printheads, the ink solvent may tend to evaporate at the nozzles causing the ink at or in the nozzles to become more viscous when the printhead and nozzles are not performing a printing operation. More viscous ink at the nozzle area tends to plug the nozzle directly affecting the performance of the printhead and printing quality. Some systems or methods for maintaining or restoring nozzle function include capping the nozzle plate, wiping the printhead with an elastomeric blade and spitting ink through the nozzles, all of which are performed when the printhead is not performing a printing operation.
Printing systems incorporating such methods typically include printheads that move back and forth on a carriage during printing operations, and the printheads are moved to a station when printing operations are stopped or suspended. Capping the nozzle prevents fluid evaporation in the nozzles and the formation of the viscous plug. Wiping the nozzle plate with the elastomeric blade clears the nozzles of the viscous plugs and dried ink residue. Spitting processes flush ink from the nozzle to clear the fluidic column of viscous ink in the nozzle including the ejection chamber. However, such processes can not be practically used in printing systems for which a printhead remains stationary during printing and does not move on a carriage during printing. Wiping or spitting methods can foul the printing medium and area surrounding a print area. In production line printing for printing bar codes, dates or other data on product packaging, the wiping and spitting techniques may interrupt a production line. In addition, the printheads for stationary printing systems in some instances are positioned so close to the print medium a cap is difficult to place on the nozzle plate.
The wiping and spitting processes may be effective for clearing the nozzles of the viscous plugs, but are inherently wasteful because ejected ink is not used for printing. In addition, printing systems monitoring an ink volume available for printing by counting ink drops ejected from the printhead may not factor ink used during cleaning operations. Accordingly, a remaining volume of ink may be over estimated and an ink cartridge may be commanded to perform printing operations with an insufficient amount of remaining ink to perform or complete a printing operation. This may lead to dry firing at the nozzles of the printhead, which may damage the printhead. In addition, an over-estimation of remaining ink volume may result in the printing system missing codes or prints on the packaging in production line printing.
Both U.S. Pat. No. 5,329,293 and JP 57061576 disclose printheads incorporating piezoelectric elements activated to discharge ink drops for printing responsive to a first signal from a controller. A second sub-firing, or voltage signal that is below a threshold voltage signal required for discharging ink, activates the piezoelectric elements to prevent clogging of ink in the nozzle. In addition, U.S. Pat. No. 6,431,674 (the “'674 patent”) discloses an inkjet printhead that minutely vibrates an ink meniscus at nozzle openings before or after a printing operation to prevent clogging of the printhead nozzles. More specifically, the '674 patent discloses an inkjet printhead of the type that utilizes the above-described piezoelectric transducers and ejection chambers, referred to as a pressure generating chamber. The printhead includes a plurality of the pressure generating chambers wherein each chamber is associated with a nozzle and each chamber has its own transducer. The piezo-transducers are activated to pressurize their respective chambers to eject ink drops from the chamber for printing. In addition, during printing inactivity, each piezo-transducer may pressurize their respective chamber to vibrate the meniscus to an extent insufficient to eject an ink drop. Because the transducer is used to pressurize the chamber for both ejecting ink and minutely vibrating the meniscus, the transducer is activated for a plurality of successive timed intervals to avoid fatiguing the transducer.
Such above-described piezo-transducer systems can not be practically incorporated in thermal inkjet printheads. Incorporating a piezoelectric transducer for each print cartridge would be cost prohibitive for manufacturing thermal inkjet cartridges or printheads. In addition, the resistive heaters incorporated in thermal inkjet printheads may not practically be used to oscillate the meniscus without ejecting ink as compared to the piezoelectric ink ejection technologies. In thermal inkjet printheads, a voltage is applied to a resistive heater associated with each firing chamber and nozzle and heats the ink in the firing chamber causing the rapid expansion of an ink bubble forcing an ink drop through the nozzle. A threshold voltage at which an ink drop may or may not be ejected from a thermal inkjet printhead is far less predictable as compared to the piezo-transducer inkjet printheads. Indeed, in printing systems incorporating thermal inkjet printheads an algorithm is used to estimate the voltage necessary to discharge ink drops. The algorithm considers such parameters such as physical properties (vapor pressure) of the ink used and dimensions of the ink channels, firing chambers and nozzles. Once the threshold voltage is determined, the algorithm is configured to select a voltage that is a predetermined percentage over the calculated threshold to ensure that ink drops will be ejected when voltage signals are applied to the resistive heaters. Application of voltage at or below a threshold voltage may or may not oscillate a meniscus, or it may cause an ink discharge. In addition, heating the ink in a firing chamber when printing has stopped or been suspended may cause ink in the firing chamber to dry and clog the nozzles.